1. Field of the Invention
The present invention relates to a deflection yoke used in a CRT (cathode ray tube) and a yoke core used for the deflection yoke, particularly relates to a deflection yoke capable of readily compensating misconvergence and a yoke core having precise dimensions and excellent magnetic characteristics with a less core loss such as an eddy current loss.
2. Description of the Related Art
In a color CRT (cathode ray tube) display device used as a display device of a computer such as a personal computer and a computer network, or as a display device for a high definition picture, there is required a high precision display performance with a less color deviation and a less geometrical distortion.
Therefore, in a deflection yoke system (referred to as deflection yoke) for generating magnetic fields to deflect electron beams in both horizontal and vertical directions, there is required a high precision performance of the generated magnetic fields to meet a required specification.
FIG. 1 is a perspective view of a CRT mounted with a deflection yoke.
As shown in FIG. 1, a CRT as an enclosed tube generally comprises a panel 1, a funnel 2 and a neck tube 3. Further, the deflection yoke 4 generally comprises a horizontal deflection coil (not shown here), a separator (not shown here) made of a plastic material, a vertical deflection coil (not shown here) and a deflection yoke core 5 (referred to as yoke core). The yoke core 5 is mounted so as to cover outer portions of the horizontal and vertical coils.
The deflection yoke 4 is mounted on the funnel 2 which is inserted from a distal end of the neck tube 3. The deflection yoke 4 deflects the electron beams emitted from the electron guns provided in the neck tube 3.
The funnel 2 nearby the distal end of the neck tube 3 has a circular cone to allow a good productivity of the CRT. The cross section of the funnel 2 at any position is made to be circular, and a center of the cross section coincides with an axis of the CRT.
Generally, the yoke core 5 of the deflection yoke 4 which is mounted on the funnel 2, has a circular cone shape corresponding to the shape of the funnel 2.
The yoke core 5 having the circular cone shape is formed as follows.
FIG. 2 is a section showing a metal mold for producing a yoke core, and
FIG. 3 is a plan view showing a lower metal mold of the metal mold shown in FIG. 2.
As shown in FIG. 2, on a supporting base 10 defining a receiving hole 9, there is fixed a lower metal mold 11. As shown in FIG. 3, there is formed a pair of projecting ribs 15 on an inner wall of the lower metal mold 11 to face each other with respect to a center line 11a of the circular section of the lower metal mold 11 for forming separation grooves 14 on the yoke core 5 to allow the yoke core 5 to be separated into two pieces after molded.
Further, there is formed a pair of projections 15, 15 for forming a pair of attachment grooves on the yoke core 5 on both sides of each of the projection ribs 14, 14. In the receiving hole section 9 formed at a center thereof, a part of an upper metal mold 13 is snugly fitted.
Between the lower metal mold 11 and the upper metal mold 13, a ferrite magnetic powder 12 made of Mg--Zn, Ni--Zn or Mn--Zn is poured, and the magnetic powder 12 is molded into a predetermined shape by pushing the upper metal mold 13 downward in a direction shown with an arrow A1 as shown in FIG. 2.
A primary product for the yoke core 5 mentioned above is referred to as a molded product 5' hereinafter.
FIG. 4 is a perspective view of a molded product for the yoke core; and
FIG. 5 is a section along an A--A line shown in FIG. 4.
The molded product 5' is shown in FIG. 4. After sintered, the molded product 5' turns into the yoke core 5. As mentioned hereinafter, when the molded product 5' is sintered, the dimensions thereof are slightly decreased because of contraction. In FIG. 4, both the yoke core 5 and the molded product 5' are shown with an identical figure by neglecting the change of dimensions after sintered.
Referring to FIG. 4, there are formed separation grooves 6, 6 and attachment grooves 7, 7 formed at both sides of each separation groove 6 for attaching metal fittings 8, 8 in the molded product 5' (yoke core 5). As shown in FIG. 5, an inner wall 5a' (5a) of the molded product 5' (yoke core 5) has a cone shape.
After sintered, the molded product 5' (yoke core 5) are separated into two parts by making use of the separation grooves 6, 6. Then, the two parts are mounted on an outer surface of an assembly of the horizontal and vertical coils which are mounted on an outer and inner surface of a separator (not shown) having a cone shape as mentioned hereinafter, and are integrally joined by inserting the metal fittings 8, 8 into the attachment grooves 7, 7.
Next, a description is given of a process for forming the yoke core 5 by sintering the molded product 5'.
FIG. 6 is a section of the molded product before sintered;
FIG. 7 is a section of the molded product after sintered;
FIG. 8 is a perspective view of a sintering holder, and
FIG. 9 is a section along a B--B line shown in FIG. 8.
As shown in FIG. 6, the molded product 5' is sintered being mounted on a sintering holder 16 in such a manner that the molded product 5' Is supported in a line contact at a peripheral portion B having a small diameter of the cone shape by an edge 16a1 which defines an opening 16a defined as a holding portion in the sintering holder 16.
After sintered, the dimensions of the molded product 5' are decreased by 15-20% because of contraction. Thus, the molded product 5' moves downward contacting with the edge 16a1 of the opening 16a, resulting in that the molded product 5' is supported at a peripheral C having a large diameter by the edge 16a1 thereof as shown in FIG. 7.
In the yoke core 5 mentioned above, there are problems as follows.
(1) It is difficult to obtain the yoke core 5 having precise dimensions. PA1 (2) It is difficult to obtain the yoke core 5 having a complex shape such as an elliptical cone shape, a rectangular cone shape and one having an irregular wall on its inner wall except for the circular cone shape. PA1 Z1: C.sub.2 .about.C.sub.16 alkylene group, phenylene group, aralkyl group, alkarilene group, --(CH.sub.2 CH.sub.2 --O).sub.n --CH.sub.2 --CH.sub.2 -- (n: integer 1-50), and PA1 Z2: C.sub.1 .about.C.sub.6 alkylene group having at least one selected from a group of a straight chain and a branched chain.
As to (1), one of the reasons that the precise dimensions are not obtained, is that the molded product 5' has the separation grooves 6 and attachment grooves 7. As explained referring to FIG. 2, when the molded product 5' is formed by pouring the magnetic powder 12 between the lower and upper metal molds 11, 13 and by being pressed with the upper metal mold 13, there occurs an uneven density distribution in the magnetic powder 12 because the density of portions nearby the separation grooves 6 and the attachment grooves 7 is different from that of other portions. Thus, when the molded product 5' is sintered, there occurs a stress between the portions nearby the separation grooves 6 and the attachment grooves 7 and other portions caused by the uneven density. This causes a problem that the yoke core 5 has a shape different from a true circle in a section because the yoke core 5 is prone to become elliptical due to the stress occurring in one direction in the process mentioned above.
Another reason that the precise dimensions are not obtained, is due to the sintering holder 16.
Specifically, when sintered, the dimensions of the molded product 5' are decreased by 15-20% by the material contraction. Thus, the molded product 5' softened by heating slides downwards while contacting with the edge 16a1 of the opening 16a. Thus, the cone shape of the molded product 5' is mostly copied from the shape of the edge 16a1 of the opening 16a of the sintering holder 16.
Incidentally, the sintering holder 16 is made of a ceramic having a heat resistance temperature of not less than 1300.degree. C., and is formed by sintering the molded ceramic powder in a high temperature. Thus, the sintering holder 16 is used as it is without a further work after sintered because of its high hardness and thus, saving an expensive cost for working if applied.
FIG. 10 shows a measured shape of an edge of an opening defined as a holding section in the sintering holder.
In FIG. 10, the measured shape of the edge 16a1 of the opening 16a is represented with a real line and an ideal shape of the edge is represented with a dotted line. In FIG. 10, plural scales are radially provided to show distances from a center 0 of the holding section (opening 16a), wherein one scale represents a distance of 20 m.mu..
As seen from FIG. 10, it will be understood that the measured shape of the edge 16a1 is deviated from the ideal one.
The cone shape of the molded product 5' when formed is largely subjected to the shape of the edge 16a1 of the opening 16a as mentioned in the foregoing.
FIG. 11(A) shows a measured section nearby the small diameter of the yoke core along a direction intersecting an axis of the CRT on which the yoke core is mounted;
FIG. 11(B) shows a measured section nearby the position shown as B of the yoke core along the direction intersecting the axis after sintered in the process shown in FIG. 6; and
FIG. 11(C) shows a measured section nearby the position shown as C of the yoke core along the direction intersecting the axis after sintered in the process shown in FIG. 6.
As seen from FIGS. 11(A) to 11(C), as a result, the sectional figures of the yoke core 5 are different at their respective positions, and have no regularity.
Accordingly, a magnetic field generated from the deflection yoke employing such a yoke core 5 is individually different, resulting in a cause of a color deviation of the picture.
As it is impossible to obtain precise dimensions by only sintering the molded product 5', the yoke core 5 is preliminarily formed to be larger than actual dimensions to allow the yoke core 5 to be worked, resulting in an increase of a production cost.
In order to solve these problems, there are proposed techniques for solving them in Japanese Patent Publication 57-11092, Japanese Patent Publication 5-15023, Japanese Patent Laid-open Publication 6-215970, and Japanese Patent Laid-open Publication 6-325961. However, they are far from the actual resolution, in particular, in the field of the high definition picture display device.
Further, in the deflection yoke mounted on the CRT having plural electron guns in line, the horizontal deflection magnetic field distribution by the horizontal deflection coil has a pincushion shape and the vertical deflection magnetic field distribution by the vertical deflection coil has a barrel shape. Thereby, the misconvergence should be theoretically eliminated.
This deflection yoke is called as self-convergence deflection yoke.
Actually, however, it is difficult to obtain such an ideal characteristic based on the theory because of the construction of the CRT, the constructive limitation of the deflection yoke, and dispersion in production, resulting in generations of many kinds of misconvergences.
As examples of the misconvergence, there is misconvergence, so-called X.sub.H or Y.sub.H.
FIG. 12 is a schematic view for explaining a misconvergence X.sub.H and Y.sub.H, and
FIG. 13 is a schematic view for explaining a VCR narrow.
As shown in FIG. 12, the misconvergence X.sub.H is defined as a phenomenon that B (blue color) and R (red color) electron beams are not converged at the same point in distal end sides of a picture in an X axis (horizontal axis) direction of the picture, resulting in an axis deviation in the horizontal direction. The misconvergence Y.sub.H is defined as a phenomenon that each of the color (R, G, B) electron beams is not converged at the same point in distal end sides of the picture in a Y axis (vertical axis) direction of the picture, resulting in an axis deviation in vertical direction.
Thus, the misconvergences X.sub.H, Y.sub.H are compensated by using compensation magnetic plates made of permalloy or silicon steel. The compensation magnetic plates are attached on an separator provided on the side surface of the electron gun so as to be at right angles (the X axis) or in parallel (the Y axis) to the array of electron guns.
In the self-convergence saddle deflection yoke employing saddle deflection coil as the horizontal and vertical deflection coils, the vertical deflection magnetic field forms a barrel magnetic field. Thus, as shown in FIG. 13, there occurs a phenomenon, so-called VCR narrow, wherein an amount of deflection of the G electron beam is decreased compared with those of the R and B electron beams. This misconvergence can not be compensated by a combination of the CRT and the deflection coils because of a constructive limitation. Accordingly, the misconvergence is compensated by flowing a compensation current to a VCR compensation (coma compensation) coils.
Here, the description is given of the construction of the deflection yoke in the prior art referred to FIGS. 14 and 15.
FIG. 14 is a partially broken section showing a deflection yoke mounted on the CRT; and
FIG. 15 is a right side view of FIG. 14.
Referring to FIG. 14, a deflection yoke 108 generally comprises a separator 101, a pair of saddle type horizontal deflection coils 102 provided on an inner surface of the separator 101, a pair of saddle type vertical deflection coils 103 on an outer surface of the separator 101 and a yoke core 104 to cover both the horizontal and vertical deflection coils 102, 103 as mentioned in the foregoing.
As shown in FIG. 14, the separator 101 has a circular cone shape extended so as to be wider from a side of a neck tube 109N of the CRT 109 to a front funnel 109F thereof. The separator 101 comprises a rear cylindrical portion 101R at an distal end thereof for accommodating a rear bent-up portion of the horizontal deflection coils 102, an attachment portion 101P extended from the rear cylindrical portion 101R and a front cylindrical portion 101F at a side of the front funnel 109F for accommodating a front bent-up portion of the horizontal deflection coils 102. The deflection yoke 108 is mounted on the CRT between the front funnel 109F and the neck tube 109N and fixed to the CRT 109 by using a band 105 and the attachment portion 101P. The R, G, B electron beams emitted from the electron gun 110 provided in the neck tube 109N are deflected by the deflection yoke 108.
Further, as shown in FIG. 15, on a back surface 101RP of the rear cylindrical portion 101R of the separator 101, there is formed a pair grooves 111 at positions close to the neck tube 109N interposed therebetween on an X axis of the CRT 109 for inserting a pair of first compensation magnetic plates 106 for compensating a misconvergence X.sub.H. Further, a pair of VCR compensation coils 107 is provided on the back surface 101RP close to the neck tube 109N interposed therebetween on the Y axis of the CRT 109. Furthermore, a pair of second compensation magnetic plates 112 is provided at positions close to the neck tube 109N interposed therebetween on the Y axis for compensating a misconvergence Y.sub.H.
FIG. 16 is a schematic view showing an example of an unsymmetrical horizontal magnetic field;
FIG. 17 is a schematic view showing another example of an unsymmetrical horizontal magnetic field;
FIG. 18 is a schematic view showing an example of the misconvergence X.sub.H according to the unsymmetrical horizontal magnetic field shown in FIG. 16; and
FIG. 19 is a schematic view showing another example of the misconvergence X.sub.H according to the unsymmetrical horizontal magnetic field shown in FIG. 17.
In FIGS. 16, 17, there are shown examples of unsymmetrical magnetic fields with respect to right and left directions. Thereby, the misconvergence X.sub.H occurs in such a manner that the B electron beam and the R electron beam are not converged at the same point in both distal end portions of the picture in the X axis direction, resulting in an axis deviation in the X axis direction as shown in FIGS. 18, 19, or resulting in that an amount of deviation between the R electron beam and the B electron beam at a right side is different from that at a left side.
FIG. 20 is a plan view showing a compensation magnetic plate; and
FIG. 21 is a schematic view showing a state where the misconvergence X.sub.H is compensated.
A compensation magnetic plate 106 made of permalloy or silicon steel as shown in FIG. 20 is inserted into the groove 111 provided along the X axis from a direction A or a direction B shown in FIG. 15. Two pieces of plates 106 may be inserted into the groove 111 from the directions A and B. Thereby, the unbalance of the horizontal magnetic field distribution is compensated with respect to the right and left directions by making use of a local cancellation of the magnetic field distribution or a change thereof caused by the compensation magnetic plate 106.
Thereby, as shown in FIG. 21, the misconvergence X.sub.H is compensated so that the B and R electron beams are converged at the same point at both distal end portions of the X axis.
Here, the compensation magnetic field caused by the compensation magnetic plate 106 tends to depend on an volume of the compensation magnetic plate 106. Thus, the larger the volume thereof becomes, the larger the compensation magnetic field becomes.
Accordingly, as shown in FIG. 20, the compensation magnetic plate 106 has an rectangular shape of a long side, 106B and a upper short side 106C and a lower short side 106D, and there is formed an inner arch surface 106A having the same radius of curvature as that of the neck 109N of the CRT 109. Thereby, it is possible to effectively cancel or change the horizontal deflection magnetic field.
On the other hand, the misconvergence Y.sub.H and the VCR narrow can be compensated by a combination of the compensation coil 107 and a VCR compensation circuit (not shown) and by causing a compensation current to flow through the VCR compensation coil 107.
Further, the misconvergence Y.sub.H can be also compensated by providing a soft ferromagnetic plate 112 made of silicon steel at a upper or a lower predetermined position along the X axis on a back surface 101RP of the rear cylindrical portion 101R of the separator 101.
FIG. 22 is a schematic view showing a R, G, B misconvergence caused by the horizontal deflection magnetic field distribution shown in FIG. 16; and
FIG. 23 is a schematic view showing a horizontal magnetic field distribution when the compensation magnetic plate is disposed closed to the B electron beam.
When the misconvergence X.sub.H shown in FIG. 18 occurs, the horizontal deflection magnetic field distribution holds a state shown in FIG. 16, wherein the magnetic field at the B electron beam side holds a stronger pincushion type magnetic field than that at the R electron beam side. In this state, the misconvergence pattern including the G electron beam in the both distal end portions of the picture along the X axis comes to a state as shown in FIG. 22.
In order to compensate the misconvergence X.sub.H shown in FIG. 22 by using the compensation magnetic plate 106, the compensation magnetic plate 106 is inserted into the groove 111 in the direction B. Then, as shown in FIG. 23, a part of the horizontal deflection magnetic flux .phi.H at the B electron beam side is distributed to the compensation magnetic plate 106. Thus, the magnetic flux in the B electron beam side is decreased compared with that at the R electron beam side. As a whole, the magnetic flux distribution is balanced with respect to the R and B electron beam sides so that the deviation of R/B electron beams is eliminated, resulting in the compensation of the misconvergence X.sub.H.
FIG. 24 is a schematic view showing a state neglecting an affect of an eddy current loss when the misconvergence X.sub.H is compensated by the compensation magnetic plate; and
FIG. 25 is a schematic view showing a state considering an affect of an eddy current loss when the misconvergence X.sub.H is compensated by the compensation magnetic plate.
In this case, as shown in FIG. 24, the G electron beam (the center electron beam) should be deviated from the R/B electron beams to outsides thereof in the both distal end portions of the picture in the X axis direction. Actually, however, as shown in FIG. 25, the G electron beams are deviated from the R/B electron beams to right side thereof. In addition, an amount of the deviation of the G electron beam in the left direction is larger than that of the G electron beam in the right direction. This reason is considered as follows.
FIG. 26 is a chart showing a sawtooth current flowing through the horizontal deflection coil; and
FIG. 27 is a schematic view showing an eddy current generated in the compensation magnetic plate.
A sawtooth current shown in FIG. 26 flows through the horizontal deflection coil 102. The sawtooth current has a repetition period T of a combination of a scanning term ts for scanning the electron beam from the left to the right in the picture and a return trace term ts for returning the electron beam to the left.
The repetition period T is determined by a horizontal deflection frequency. In the high definition display, a high horizontal deflection frequency is selected. The value of the return trace term tr is 1/5 as small as that of the scanning term ts, i.e., the scanning frequency is 5 times as large as that of the return trace frequency, because the electron beam has to be quickly returned to the left side of the picture.
Thus, an eddy current is generated in the compensation magnetic plate 106 at the return trace term tr. The value of the eddy current generated at the return trace term tr is larger than that at the scanning term ts, resulting in a magnetic field .phi.e as shown in FIG. 27 caused by the eddy current at the beginning.
The magnetic field .phi.e caused by the eddy current is superimposed on the horizontal deflection magnetic field, so that the effect of the compensation of misconvergence caused by the compensation magnetic plate 106 is weakened.
Especially, the horizontal deflection magnetic field close to the end portion of the deflection yoke has a strong pincushion magnetic field compared with that nearby the middle portion thereof. Thus, the G electron beam at the left side of the picture is deviated to the right side.
Accordingly, in order to prevent the deviation of the G electron beam to the right side, it is effective to eliminated the generation of the eddy current caused by the compensation magnetic plate 106. Otherwise, a different method is required to eliminate the misconvergence X.sub.H.
In order to eliminate the effect of the eddy current, it is effective to employ a magnetic plate having a low eddy current loss in the frequency band used. For instance, Mg--Zn ferrite, which is used in the deflection yoke core as mentioned in the foregoing, is used. However, the Mg--Zn ferrite has such a drawback as being weak in the mechanical strength. Thus, it is necessary to cause the thickness thereof to be thicker than those of the permalloy and the silicon steel, resulting in a limitation of the shape. In addition, the cost of the compensation magnetic plate of Mg--Zn ferrite is more expensive.
As another method, it is possible to compensate the misconvergence by employing a convergence yoke, wherein an analog or a digital compensation current is added to the convergence yoke. However, this method has a drawback of a high cost because of employing the deflection yoke and a compensation circuit. Thus, it is impossible to employ such method in the deflection yoke used in a general use.
Next, the description is given of an example of the VCR narrow compensation in the prior art.
A pair of multi-pole coils each having an E-shaped magnetic core with plural legs around which coils are wound, is arranged on an insulator provided on the side of the electron guns of the CRT in such a manner that the multi-pole coils face to each other in a direction (X axis direction) perpendicular to an extended line of the electron gun arrangement. The coils of the pair of the multi-pole coils are connected in series, and they are connected to the vertical deflection coil to allow the vertical deflection current to flow through the coils of the multi-pole coils so that VCR compensation (comma compensation) is performed.
FIG. 28 is a partially broken section which is vertical to the section of FIG. 14, showing a deflection yoke mounted on the CRT; and
FIG. 29 is a right side view of FIG. 28.
Referring to FIG. 28, a deflection yoke 207 generally comprises a separator 201, a pair of saddle type horizontal deflection coils 202 provided on an inner surface of the separator 201, a pair of saddle type vertical deflection coils 203 on an outer surface of the separator 201 and a yoke core 204 to cover both the horizontal and vertical deflection coils 202, 203 as mentioned in the foregoing.
As shown in FIG. 28, the separator 201 has a circular cone shape extended so as to be wider from a side of a neck tube 208N of the CRT 208 to a front funnel 208F thereof. The separator 201 comprises a rear cylindrical portion 201R at an distal end thereof for accommodating a rear bent-up portion of the horizontal deflection coils 202, an attachment portion 201P extended from the rear cylindrical portion 201R and a front cylindrical portion 201F at a side of the front funnel 208F for accommodating a front bent-up portion of the horizontal deflection coils 202. The deflection yoke 207 is mounted on the CRT between the front funnel 208F and the neck tube 208N and fixed to the CRT 208 by using a band 205 and the attachment portion 201P. The R, G, B electron beams emitted from the electron gun 209 provided in the neck tube 208N are deflected by the deflection yoke 207.
Further, as shown in FIG. 29, on a back surface 201RP of the rear cylindrical portion 201R of the separator 201, there are disposed multi-pole coils (VCR compensation coil) 206, 206' on a back surface 201RP of the rear cylindrical portion 201R of the separator 201 at positions close to the neck tube 208N interposed between the multi-pole coils 206, 206' so as to compensate the misconvergence VCR.
Each of the multi-pole coils 206, 206' comprises an E-shaped magnetic core 211, coils 212a to 212c (212d to 12f) each wound around a let of the E-shaped magnetic core 211.
FIG. 30 is a plan view showing a soft magnetic plate used in an E-shaped magnetic core of a multi-pole coil; and
FIG. 31 is a plan view of the multi-pole coil.
As shown in FIG. 30, a soft-magnetic plate 210 having an E-shape is formed from a silicon steel plate or a permalloy plate by punching. The E-shaped magnetic core 211 is formed by stacking a plurality of the soft magnetic plate 210.
As shown in FIG. 31, the multi-pole coil 206 (206') is fabricated by winding coils 212a, 212b, 212c (212d, 212e, 212f) around legs of the E-shaped magnetic core 211.
FIG. 32 is a schematic back view of the deflection yoke for explaining an operation of the multi-pole coils, wherein the deflection of the electron beams is performed with respect to an upper half of the picture.
Each of the coils 212a to 212f is electrically connected as follows. When the deflection of the electron beams is performed with respect to an upper half of the picture, the magnetic poles of the E-shaped magnetic core 211 of the multi-pole coil 206 are made to be S (south) pole, N (north) pole and S (south) pole in this order downwardly, and the magnetic poles of the E-shaped magnetic core 211 of the multi-pole coil 206' are made to be N-pole, S-pole and N-pole in this order downwardly. When the vertical deflection magnetic field is zero, the R, G, B electron beams emitted from the electron guns disposed in a lateral (horizontal) direction are at a position between both central magnetic poles of the E-shaped magnetic cores 211 of the multi-pole coils 206, 206'.
When the deflection of the electron beams is performed with respect to the upper half of the picture, a positive direct vertical deflection current flows through the vertical coil 203 and multi-pole coils 206, 206'. By the current flowing through the multiple-pole coils 206, 206', there are generated a first magnetic field caused between the central pole (N pole) and both end poles (S pole) in a direction shown with an arrow 216, and a second magnetic field between both end poles (N pole) and a central pole (S pole) in a direction shown with an arrow 217.
Thus, the electron beams R, G, B behave according to the above magnetic fields as follows.
The R and B electron beams are respectively situated close to the central poles of the multi-pole coil 206 and 206'. The R electron beam is affected by the first magnetic field caused by the central pole (coil 212b) of the multi-pole coil 206 and the B electron beam is affected by the second magnetic field caused by the central pole (coil 212e) of the multi-pole coil 206'. Thus, the R electron beam moves downward in a direction shown with an arrow 213 and the B electron beam moves also downward in a direction shown with an arrow 214.
Further, a third magnetic field is generated between the both end poles (N poles: coil 212d, 212f) and the both end poles. (S pole: 212a, 212c) in a direction shown with arrows 218, 219. A magnetic field generated between the central N-pole of coil 212 band the central S-pole of coil 212e is cancelled by the third magnetic field shown with the arrows 218, 219.
Thus, the G electron beam is affected by only the third magnetic field, so that the G electron beam moves upward in a direction shown with an arrow 215.
As mentioned above, the R, G, B electron beams are affected by only the magnetic fields generated in a horizontal direction. Thus, the R, G, B electron beams are deflected in upward and downward directions. This enables to compensate the misconvergence VCR narrow.
Upon compensating the VCR narrow, as the G electron beam is situated at the center of the three electron beams, an amount of deflection is apt to be small. Therefore, there may be generated other misconvergence, so-called greened loop, wherein an amount of deflection at the center portion does not coincide with an amount of deflection in the peripheral portion, so that the G color line becomes a bow shape compared with the R and B color lines. This misconvergence can be compensated by superimposing a parabola current having the horizontal deflection period modulated by the vertical deflection period.
As mentioned in the foregoing, the multi-pole coils 206, 206' are provided on the back surface 201RP of the rear cylindrical portion 201R of the separator 201 at positions close to the neck tube 208N interposed between the multi-pole coils 206, 206'. Thus, a part of the horizontal deflection magnetic field (flux) is distributed to each of the E-shaped magnetic cores 211, resulting in a generation of the eddy current in each of the E-shaped magnetic cores 211.
The horizontal deflection magnetic field is generated by causing the sawtooth current shown in FIG. 26 to flow through the horizontal deflection coil 202.
The repetition period T of the sawtooth current is determined by a horizontal deflection frequency. In the high definition display, a high horizontal deflection frequency is selected. The value of the return trace term tr is 1/5 as small as that of the scanning term ts, i.e., the scanning frequency is 5 times as large as that of the return trace frequency, because the electron beam has to be quickly returned to the left side of the picture. Thus, the value of the eddy current generated in the E-shaped core 211 becomes maximum at a beginning and an end of the return trace term tr, and is gradually decreased as the current becomes zero at the center of the picture. Then, the value of the eddy current is gradually increased as the current becomes maximum at the right side of the picture.
FIG. 33 is a schematic view showing a magnetic field generated by an eddy current of the E-shaped magnetic core.
Here, when the electron beams are deflected from the left side to the center of the picture, a magnetic field .phi.E in a direction shown with an arrow in FIG. 33 is generated by the eddy current caused in the E-shaped magnetic core 211. The magnetic field .phi.E is superimposed to the horizontal deflection magnetic field generated by the horizontal deflection coil 202 in the same direction thereof. Thus, as it comes to a position close to an end of the picture, the horizontal deflection magnetic field distribution at the rear portion becomes a strong pincushion compared with one at the front portion. Especially, this inclination is stronger at the left side of the picture (at the beginning of electron beam scan) than other portion thereof.
Accordingly, when a deviation (misconvergence X.sub.H) of a vertical R line from a vertical B line generated at respective ends of the picture in the direction of the X axis is compensated, a vertical G line is deviated from the vertical R/B line at the respective ends of the picture as shown in FIG. 25. In addition, an amount of the deviation at the left side of the picture is larger than at the right side thereof, resulting in a degradation of the compensation quality of the misconvergence. This phenomenon occurs in other type magnetic cores other than the E-shaped magnetic core.